Radio transmitting system



Dec. 8, 1936.

E. H. ARMSTRONG 2,063,074

RADIO TRANSMITTING I SYSTEM Filed Sept. 14, 1935 2 Sheets-Sheet 1' Frequency Pawer mp/ifier Daub/er Fllrer Amp/mar Half/Pl!!! Amp/If 27 30 Edwin H. Armsrronq.

, ATTORNEYS 1936. H. ARMSTRONG 2,063,074

RADIO TRANSMITTING SYSTEM Filed Sept. 14, 1935 2 Sheets-Sheet 2 6 Amp/ifude r A of 4 5m Frequencies. v

40,000 50,000 00,000 Frequency f 4 i Tube- Sysfem i /8-2/. 2 i v i 0 40,000 7 50,000 60000 Frequency I;

Combined Clmmder/sfic l6 40,000 50, 000 00,000 Frequency INVENTOR Edwin H. Armsfrang,

A TTORNEY5 Patented Dec. 8, 1936 UNITED STATES PATENT OFFICE RADIO TRANSMITTING SYSTEM' Edwin H. Armstrong, New York, N. Y.

Application September 14, 1935, Serial No. 40,542

1 Claim.

This invention relates to a method of improving the operation of the system for producing a frequency modulated wave from a source of fixed phase and frequency described in my U. S. Patent #1,941,068.

The object of the invention is to correct certain asymmetrical conditions in the modulating system so that the radiated wave may be made more symmetrical in nature and distortion at the receiver reduced to a minimum.

As explained in the patent two side frequencies corresponding to the frequency of the modulating current are created from a source of fixed phase and frequency by means of the well known carrier suppressor system and these, after changing the phase of each thru are combined with a current derived from the fixed frequency to producea resultant current whose phase is alternately advanced and retarded. By making the angle of phase shift directly proportional to the amplitude of the modulating current and inversely proportional to the frequency thereof and by multiplying it a sufiicient number of times by a frequency multiplying system there is produced a frequency modulated current.

Under certain conditions, particularly when the frequency of the initial source is low or when the frequency of modulation becomes greater than a certain percentage of the initial frequency the phase of the angle of shift is not symmetrical, due to causes which will be explained hereinafter, and it is the purpose of this specification to disclose means for correcting this so that a symmetrical phase shift and a symmetrical frequency modulated wave is produced.

Referring now to the figures which form a part of this specification, Figure I illustrates the arrangement of the system and Figure 11 the reactance characteristic of the asymmetrical part of the modulator and the characteristics of the compensation system.

Referring now to Figure I there is illustrated the transmitting system described in my U. S. Patent #1,94l,068. In this figure i represents the source of constant frequency and 2-3 an amplifier for this frequency. 4 to l2 inclusive represent a carrier suppression system for producing two side bands in accordance with modu= lations impressed by the correction system 3-41. 18 represents an amplifier whose plate circuit contains the compensation system I 9, 20, 2| which will be referred to later in this specification. 22, 23 represents the ordinary blocking condenser and grid leak for coupling the amplifier 24 to the compensation system. 24 is an amplifier exciting a doubler 25 whose output is passed thru a filter 26 whose purpose is the elimination of the fundamental frequency as described in my application for U. S. patent Serial Number 40,543, filed September 14, 1935. 21 is an amplifier 28 a doubler and 29 a filter for the succeeding frequency. 30, 3|, 32, 33, 34 are the usual amplifier, frequency multipliers, power amplifier and radiating system of a/ frequency modulation transmitter. l3 represents the input of the modulating system, M an amplifier and |5-l6 the correction system described in the patent above referred to. H represents an amplifier for supplying the corrected current to the input of the modulating transformer 6.

- Now as explained in the patent above referred to, the reactance of the coils 9 and ID are made low with respect to the impedances of the tubes 4 and 5 and the condensers 'l and 8 are adjusted to balance out even this small amount of reactance. Therefore the plate circuit in addition to being aperiodic is substantially symmetrical with respect to the side frequencies. The currents in the plate circuits are therefore symmetrical, that is for any given frequency of modulation, the amplitude of the current whose frequency is the sum of the modulation frequency and the constant radio frequency of the generator I is equal to the amplitude of the current whose frequency is the difference of these two frequencies. Therefore since these currents are equal in amplitude, the voltages induced by them in the secondary H are unequal. This follows because the induced voltages are equal to 27rfMI where f is the frequency M the mutual inductance and I the plate current. Since the currents are equal it follows that the induced voltages are proportional, respectively, to the frequencies of the sum and the difference frequencies created by modulation.

Where the frequency of the generator is high the difference is of no importance but where the frequency of the generator is of the order of 50,000 cycles then, for a modulating frequency of 5,000 cycles a 22% unbalance is created and for a 10,000 cycle modulating current a 50% unbalance is created.

It is the purpose of this specification to describe means for compensating for this asymmetry so that the current which is ultimately supplied to the frequency multiplication system is symmetrical in all respects. This is accomplished by the compensation coupling system I9, 20, 2i between the tubes I 8 and 24. In this system I9 is the ordinary coupling resistance shunted by a branch containing the resistance 20 and condenser 2| whose impedances are preferably larger than that of the resistance IS. The capacity of the condenser 2| is so selected with respect to the value of the resistance 20 that its impedance for the frequencies to be employed is low with respect to the resistance 20. The amplification of the tube l8 therefore varies as the reactance of the condenser II in the manner illustrated by curve B in Figure II, that is, inversely as the frequency. Since the amplitude of the side bands supplied to the input of the tube I! by the carrier suppression system is in direct proportion to their frequency it follows that the product of the curves A and B is a level characteristic as illustrated in curve C. The whole process can be carried out with great accuracy since 'the secondary II is unloaded and since the ratio of impedance of the condenser 2| may be made very small with respect to the resistance of 20.

The method is of particular importance when the frequency of modulation is high as in facsimile or television transmission where it is difficult to secure a low ratio of modulation to initial frequency even when the initial frequency is high and the multiplied frequency is reduced and re-multiplied.

It is, of course, obvious that many ways of compensation by combined coupling might be employed such as combined capacity and magnetic coupling in the carrier suppression system 9, III, II or the compensation system might be placed between the output of tube l2 and the resistance 3 and the phase of the frequency supplied to 3 by the tube 2 advanced or retarded in order to compensate for the 90 phase shift introduced by the compensation system. All these methods would give equivalent results but the method illustrated in the diagram is probably the simplest way of carrying out the invention.

I have described what I believe to be the best embodiments of my invention. I do not wish, however, to be confined to the embodiments shown, but what I desire to cover by Letters Patent is set forth in the appended claim.

I claim:

The combination with a system for producing frequency modulated waves, comprising a source of fixed phase and frequency, a carrier suppression modulation system for producing two side bands, and means for combining the side bands with a current derived from the source of fixed phase and frequency in proper phase relation to produce the frequency modulated wave, of means for equalizing the asymmetry produced in the side bands by the carrier suppression system, the frequency characteristic of which is substantially the converse of that of the carrier suppression system, and means for passing the side band currents through the said equalizing means, so as to produce substantial equalization of the amplitude of the side bands.

EDWIN H. ARMSTRONG. 

